Lava began to flow today from Chile's Chaitén volcano, chasing remaining residents out of a nearby town and putting the government of the affected Palena Province on high alert. The country had already been on edge following the volcano's initial eruption this past weekend, spewing hot ash, gas and smoke into the air for several days, forcing the evacuation of more than 4,200 residents and leading to the death of a 92-year-old woman who suffered a heart attack aboard a navy boat as she was being taken to Puerto Montt, about 125 miles (200 kilometers) north of the volcano. No lava flow, however, had been reported until Tuesday.

Volcanoes are openings, or vents, in Earth's crust through which magma—formed when the planet's upper mantle and lower crust melts—and gases are discharged. Once the magma percolates to the surface and begins to flow from the volcano, it is called lava. Ash, more than six inches (15 centimeters) thick in some places, had already coated houses, vehicles, trees and water supplies.

The eruption came as a surprise, given how long the volcano had been dormant—various news reports state that the volcano's last eruption was anywhere between 400 and 9,000 years ago. The eruption was the first for Chaitén volcano in recorded history and followed two days of unusual seismic activity in the zone, The Patagonia Times reported Monday.

Predicting when an eruption will occur requires measuring a number of parameters, including earthquake activity and gas emissions at the volcano. The methods are far from foolproof, so scientists are experimenting using lasers to examine changes in carbon isotopes in carbon dioxide, which might signal an influx of carbon dioxide from magma either building under or rising through the volcano.